This is a competitive renewal application to study the role an atypical homeodomain protein called Hopx that is expressed in the heart. My laboratory discovered Hopx about 10 years ago, and we have shown that it functions, at least in part, by recruiting histone deacetylases (HDACs) to transcription complexes. This work, funded by this grant, led to numerous high-profile publications, patent applications, and it has spurred our efforts in collaboration with the private sector to translate the findings and bring new therapies for heart failure to the clinic. Our mechanistic studies have shown that Hopx does not bind directly to DNA, but that it is a component of cardiac repressor complexes. Hopx is expressed by cardiac progenitor cells at early time-points of murine embryogenesis, just after Nkx2-5, at ~E8.0, and inactivation of Hopx leads to partially penetrant embryonic lethality and thin myocardium. Cardiac progenitors that express Nkx2-5 are multipotent and can produce myocardial, smooth muscle or endothelial lineages. However, Hopx-expressing progenitors are committed to the myocardial lineage. Our data suggest that fate decisions of cardiac precursor cells are biased by Hopx dose and activity. We hypothesize that Hopx is an extremely early (perhaps the earliest) marker of committed myocardial cells and that Hopx functions to reinforce the myocardial lineage choice by recruiting HDACs and other transcription cofactors to repress critical mediators of alternate fates and of the multipotent state. Understanding how myocardial fate decisions are executed and reinforced will inform our ability to enhance regenerative therapies and instruct stem and progenitor cells to produce functional myocardium.